1. Field of the Invention
This invention relates to a method for making a martensitic stainless steel seamless pipe. The seamless pipe has high corrosion resistance and is suitable for oil country tubular goods (OCTGs). In particular, the invention relates to improvements in toughness and a decrease in anisotropy of toughness.
2. Description of the Related Art
In consideration of the advance in crude oil prices and anticipated depletion of oil resources in the near future, deep stratum oil fields and highly corrosive sour gas fields are being developed all over the world.
These oil and gas fields generally spread out at very deep layers and in severely corrosive environments at high temperatures containing CO2, Cl− ions and the like. Thus, OCTGs used in these fields must have high toughness and high corrosion resistance. In general, under severe corrosive environments containing such CO2, Cl− ions and the like, martensitic stainless steel seamless pipes with high CO2 corrosion resistance containing 13% chromium are primarily used.
Martensitic stainless steel seamless pipes are generally produced by the following process: A raw steel material is heated to a temperature capable of piercing, and subjected to piercing using a piercing mill and elongating using a mandrel mill or plug mill to form an original pipe. The original pipe is reheated to an austenitic temperature range and subjected to finishing rolling using a hot stretch reducing mill or a sizing mill. After air-cooling, the composition of the seamless pipe comprises martensite. The seamless pipe is subjected to quenching from the austenitic temperature range and tempering at a temperature below the AC1 transformation point if higher strength and higher toughness are required.
Oil well pipes used in deteriorating well environments must have higher mechanical properties, such as higher toughness at low temperatures and higher resistance to sulfide stress cracking.
In order to satisfy such requirements, for example, Japanese Unexamined Patent Application Publication No. 1-123025 discloses a method for making a martensitic stainless steel seamless pipe. This method includes the steps of piercing and rolling a martensitic stainless steel slab at a temperature of 1,050° C. to 1,250° C.; cooling the rolled pipe at a cooling rate of 30° C./min to at least 500° C. and further cooling the pipe to a temperature below the martensite transformation temperature to form a steel structure containing at least 80% of martensite; reheating the pipe to a temperature between (Ac1 transformation point −200° C.) and Ac1 transformation point and finishing-rolling the pipe at a reduction in area of at least 5%; maintaining the pipe at the final finishing-rolling temperature or reheating the pipe to a temperature below the Ac1 transformation point immediately after the finishing rolling step, and then cooling the pipe by spontaneous or forced air cooling. Alternatively, after the step of forming the martensitic structure, this method may include the steps of reheating the pipe to a temperature between the (Ac1 transformation point −200° C.) and the Ac1 transformation point, finishing-rolling the pipe at a reduction in area of at least 5%, and then cooling the pipe by spontaneous or forced air cooling; reheating the pipe to a temperature below the Ac1 transformation point immediately after the finishing rolling step, and then cooling the pipe by spontaneous or forced air cooling.
However, the seamless pipe produced by this method has the following problem: Since the pipe is rolled at a non-recrystallization temperature range, the structure is elongated in the rolling direction. As a result, the toughness and corrosion resistance of the seamless pipe are high in the rolling direction, but low in the circumferential direction perpendicular to the rolling direction. In other words, the seamless pipe exhibits noticeable anisotropy in mechanical properties.